Interleukin-33 (IL-33), also known as NF-HEV, is a 30 kDa protein and a member of the IL-1 family. This pro-inflammatory cytokine is produced by a variety of cells but is primarily expressed by endothelial and smooth muscle cells in the vasculature and airways. IL-33 binds to the ST2 receptor on target cells which results in the activation of multiple signaling pathways including NF-κB and MAP kinases. Studies have shown that IL-33 promotes T helper type 2 (Th2) cell-mediated immune responses and upregulates production of Th2 associated cytokines including IL-4, IL-5, and IL-13. In vivo administration of this cytokine in mice results in eosinophilia and splenomegaly as well as induced severe pathological changes in the lungs, gastrointestinal tract, and other mucosal organs.